A boost converter is a power converter where a smaller input DC voltage is increased to a desired DC voltage level at the converter's output. A typical prior art boost converter 10 is shown in FIG. 1. Boost converter 10 has input terminals 2, 4 for enabling an input voltage Vin to be coupled to converter 10, and output terminals 6, 8 where the output DC voltage is provided. The boost converter 10 includes an inductor 18, to which the input voltage Vin is coupled, that is in series with a boost diode 16 connected to an output capacitor 12 across which the load (not shown) is connected at terminals 6, 8. A transistor switch 14 is connected to a node 15 between the inductor 18 and boost diode 16 and a ground return line 20 to provide regulation of the output voltage. The switch 14 is typically a MOSFET having a control input, a drain and a source terminal. A control circuit 42 (details not shown) is coupled to the control input for providing a control signal for controlling the timing of the on and off transition of the switch 14. The control circuit 42 typically includes a pulse width modulation circuit (PWM). In operation, when the switch 14 is on, the inductor current in inductor 18 increases, storing energy in its magnetic field. When the transistor switch 14 is off, this energy is transferred via the diode 16 to the output capacitor 12 and the load. Drawbacks of such conventional boost converter circuits include the creation of switch voltage and current stresses, which result in low efficiency power conversion.
FIG. 2 shows a prior art boost converter 20 having a snubber circuit 24 added to the boost converter 10 of FIG. 1. The snubber circuit 24 is designed to suppress the spike generated at the drain of MOSFET 14 caused by the large current pulse and the leakage (parasitic) inductance in the circuit when MOSFET 14 is turned off and to transfer this energy to the output. The snubber circuit 24 includes a series combination of a capacitor 22 and a diode 36 connected across a diode 16. The capacitor 22 is connected at one end to the junction of inductor 18 and diode 16 at a node 25. Snubber circuit 24 also includes a series combination of a diode 26 and an inductor 28 connected between the junction of the capacitor 22 and the diode 36 and the negative output terminal 8.
In operation, when the MOSFET switch 14 in boost converter 20 switches from ON to OFF, the spike that is created at node 25 is clamped due to the voltage across capacitor 22 which causes turn on of diode 36 before diode 16 is turned on. Thereafter, the voltage at node 25 will become slightly higher than the output voltage at terminal 6 causing diode 16 to conduct and transfer the energy stored in inductor 18 during turn ON and clamp the voltage of node 25 to Vout. Capacitor 22 and inductor 28 form a resonant network that, through half resonant action after MOSFET 14 is turned ON, reverses the polarity of the voltage across capacitor 22.
A drawback of the circuit in FIG. 2 is that the voltage across capacitor 22 cannot be controlled at turn on and so, after the half resonant period, the reverse voltage is less than before. The voltage across capacitor 22 cannot be controlled on turn on of MOSFET 14 because the energy stored in inductor 28 is not sufficiently controlled. Consequently, the performance of the circuit in FIG. 2 is not suitable, particularly in high power applications. For one exemplary converter in an experiment setup, just after the MOSFET 14 turns off, the voltage across capacitor 22 becomes +50V, and on turn on of MOSFET 14, after the half resonant period, the voltage across capacitor 22 becomes −40V. The voltage loss is due to the resonant choke, i.e., inductor 28 and diode 26. Thus, although an improvement over converter 10 of FIG. 1, converter 20 has the similar drawback of having an unacceptably high voltage spike (>Vout) at the drain of MOSFET 14 for high current applications so as to require higher voltage rated MOSFETS with higher Rdson and corresponding higher conduction losses.
A need therefore exists for a snubber circuit and power converter topology which solves the above described drawbacks of the known circuits by controlling the voltage across the resonant capacitor at turn on of the main MOSFET so as to enable the use of lower voltage and lower Rdson rated MOSFETs and to provide increased efficiency by reducing conduction losses.